Temperature control system for heating apparatus



Nov. 13, 1951 c. K. STROBEL 2,575,073

TEMPERATURE CONTROL SYSTEM FOR HEATING APPARATUS Filed NOV. 5, 1947 2 SHEETS.SHEET l 3? 44 I vvvv :1?

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i atented Nov. 13, 1951 TEMPERATURE CONTROL SYSTEM FOR HEATING APPARATUS Charles K. Strobe], Pittsburgh, Pa., asslgnor to Robertshaw-Fulton Controls Company, Youngwood, Pa., a corporation of Delaware Application November 5, 1947, Serial No. 784,161

3 Claims. 1

This invention relates to temperature control systems and, more particularly, to electrical systems in employing thermally sensitive resistors for the control of temperature in ovens, rooms or other enclosures.

It has long been known that certain metallic oxide semi-conductors possess high negative temperature coeiiicients of resistance and will therefore decrease in resistivity with increase in temperature afiecting the material. The temperature variation may be accomplished externally of the resistor by changes in ambient temperature or internally by the heating effect of current passing therethrough. Many different types of thermal resistors are available including discs or washers, rods and beads of nickel-manganese oxide, silver sulphide and other materials and usually mounted with electrodes and leads. The rod type is usually coated with glass or other electrical insulating material while the bead type is mounted in sealed glass bulbs. The devices remain stable 'in resistance value when hot or cold, the former being a valuable characteristic in the application herein described.

It is apparent, therefore, that a simple circuit employing thermal resistance means of the indicated nature would serve to eliminate the relatively bulky and expensive temperature control elements now universally employed, for example, in domestic gas and electric ranges. The rod-and-tube thermostat or the liquid-type employing a bulb, bellows and capillary element now largely in use require considerable skill and care in manufacture and occupy valuable space when installed in the range or oven. By

comparison, the thermally sensitive resistors contemplated by this invention are of insignificant size and correspondingly uncomplicated and inexpensive to manufacture.

With these and other objects and advantages which will become apparent hereinafter, the invention consists in a novel arrangement of parts now to be described and which is illustrated in the accompanying drawings, wherein:

Fig. 1 is a schematic view of a temperature control system for a domestic gas oven embodying the invention;

Fig. 2 is a similar schematic view of a temperature control system for a domestic electric oven embodying the invention;

Fig. 3 is a schematic view of the temperature control system for central heating apparatus embodying the invention; and

' Fig. 4 is a'schematic view of a modification of part of the system shown in Fig. 3.

Referring more particularly to the embodiment disclosed in Fig. 1 of the drawings, the tempera.- ture control system is applied to a domestic gas range having an oven indicated in broken lines in the drawing and containing a main burner l0 and a pilot burner I2 in lighting proximity there,- to. The main burner I0 is supplied with fuel by a main fuel pipe ll wherein the flow of fuel is under control of a main gas cook l6 and also of an electrically operable means in the form of an electromagnetic or solenoid valve l8 whichis biased to a closed position.

The main valve I6 is manually operable between off and on positions as indicated on the usual handle or dial l9 by which the cock is rotated. In accordance with this invention the dial l9 also carries temperature indicia 20 for various settings corresponding to selected values of the oven temperature. Only one such temperature indicia 20 is indicated in the drawing as such dial markings are well known and further illustration is deemed unnecessary. The flow of fuel to the pilot burner I2 is also under control of the main valve l6 by the connection of one end of a conduit 22 to the pilot burner l2 and to the main valve 16 at the opposite end. It will be understood that a constant-burning pilot burner could be employed if desired by connecting the said opposite end of the conduit 22 to the main fuel pipe H ahead of the main valve IS.

The control circuit for the main burner l0 includes the thermal resistance means 24 of this invention and which is in the form of a semiconductive material having a high negative ternperature coeiflcient of resistance for decreasing in resistivity in accordance with an increase in temperature ambient thereto. In this embodiment, the thermal resistance element 24 is located in the oven being heated by the main burner l0 and will, in conjunction with other elements to be described, control the temperature of the oven in accordance with a selected value on the dial l9.

A second thermal resistance means 28 is utilized in the control circuit to be described and takes the form of a rheostat or variable resistor of any known and suitable form. A movable arm 28 is cooperable with the resistance means 26 in a well-known manner to vary the resistivity setting over a desired range corresponding with the temperature inclicia 20. As shown schematically in Fig. 1, the thermal resistance means 26 is located beneath the dial I9 to which the arm 28 is secured at one end for movement across the surface of the resistance means 26 in good electrical contact therewith. In the "of!" position of the dial l9, that is, when the fuel cock is closed, the arm 28 is out of operative contact with the resistance mean 26. The resistance means 26 may also be of the type havin thermal resistance characteristics for a purpose to be apparent hereinafter and preferably has a positive temperature coeilicient of resistance for increasing in resistivity in accordance with the passage of electric current therethrough.

The connection of the various electrically energized elements of the system in the control circuit will be described in connection with the operation of the system,.whlch now ensues. Assuming that the oven is cold and both the main burner l and the pilot burner I 2 are extinguished, the system is placed in operation by turning the dial l9 from the oil? position shown to a selected temperature setting shown by the indicia 20. During this operation, the arm 28 is rotated relative to the adjustable resistance means 26 a predetermined amount corresponding to the selected temperature of the oven. Consequently, the resistivity setting of the adjustable resistance means 26 is adjusted and will remain at such value as is predetermined by its temperature-current characteristics.

A circuit is now established which may be traced as follows: line wire 30, wire 32, movable arm 28, resistance means 26, wire 34, wire 36, coil of the solenoid valve l8, wire 38 and wire 40 back to line wire 42. The resistance means 24 is connected in parallel mesh circuit with the coil of solenoid valve l8 by wire 44 from one end of resistance means 24 to wire 34 and wire 46 from the opposite end of the resistance means 24 to wire 40. It will be observed that the adjustable resistance means 25 is in series circuit with the parallel mesh consisting of the coil of the solenoid valve l8 and the resistance means 24.

The resistance means 24 is unheated at this time and has a relatively high resistance so that suillcient current flows through the coil of the solenoid valve I8 to cause actuation of this valve against its bias to open position. Consequently, fuel is admitted to the main burner l0 and the pilot burner l2 and ignition of these burners can be conducted.

As the temperature in the oven increases, the resistance means 24 becomes heated accordingly and its resistance correspondingly decreases causing an increase in the circuit current and voltage drop across the adjustable resistance means 26. This increase in voltage drop is enhanced by the positive temperature coefllcient of this element as will be apparent. The net result is that the voltage drop across the coil of the solenoid valve 18 is decreased and, at a definite temperature of the resistance means 24, the valve l8 closes to shut-off the supply of fuel to the main burner III.

The system is of the recycling type as is necessary for temperature control in the oven. Thus, in the cooling cycle the action previously described is reversed and at a definite temperature of the resistance means 24 the supply of electric energy to the coil of the solenoid valve I8 is of suflicient value for operation of the valve I8 to open position. It is apparent, therefore, that the arrangement provides for maintaining the oven temperature in substantial accordance with the setting on the dial l9 throughout the operation of the oven.

In the relatively simple circuit disclosed in Fig. 1 of the drawings the solenoid valve I8 is operated directly by changes in resistivity of the resistance with variations in the temperature.

means 24 in cooperation with the resistance means 28. A reduction in the power consumption of the system could, of course, be obtained by the interposition of a relay device between the solenoid valve l8 and the resistance means 24. Such a relay-operated system lends itself particularly to the control of electric heating elements as is disclosed in the embodiment shown in Fig. 2. In this arrangement the temperature control system is applied to an electrically heated oven having electric resistance heating means 48 located therein. In this and succeeding embodiments', similar reference characters to those used in connection with Fig. 1 have been employed for similar parts. The resistance means 24 of this invention is positioned in the oven to respond to the temperature condition to be controlled by varying in resistivity in accordance The energization of the heating means 48 is controlled by a relay 50 having a coil 52 and switching means in the form of an armature 54 operable thereby between open and closed positions relative to a fixed contact 56. Thus, upon variation in the energy value supplied to the coil 42 the armature 54 will be moved relative to contact 56 between its biased or released position and its attracted or contact closed position.

.Manually operable control means it for controlling the fiow of energy in the system include the handle or dial l9 carrying the ofl and on markings and the temperature indicia 20. The adjustable resistance means 25 is cooperable with a movable arm 60 carried by the handle it to establish the resistivity settings of the adjustable resistance means 25 over the predetermined range corresponding to selected oven temperatures. In the off position of the handle is the movable arm 60 occupies a position intermediate opposite ends of the adjustable resistance means 26 as indicated in Fig. 2 of the drawings.

In order that one side of the line to the relay coil circuit may be broken in this ofl" position of the dial l9, suitable switching means are provided for cooperation with the movable arm 84. Such switching means takes the form of an ordinary snap-switch device 52 having the operating button 64 thereof engageable by the movable arm 60 in its movement between the "01! and "on positions of the handle 19. Thus, in the "oil" position of the dial IS the snap-switch 82 will also be in o position but will be operated to an on position when the dial i9 is rotated to- Yard the "on position thereof.

As indicated, the snap-switch arrangement will serve to open one side of the line circuit to the a control circuit but the circuit of the heating means 48 is opened only by the positioning of the arm 60 in the off" position of the dial is. Consequently, it will be apparent that the other side of this line circuit could also be opened simultaneously if desired by the provision of a. second switching means similar to the snap switch 62 at the appropriate location. 0n the other hand, if it is desirable to open only one side of the line for all circuits in the "off" position of the handle I9, then the snap-switch 62 could be dispensed with and the deenergization effected by the mere positioning of the movable arm 80 in "01? position relative to the adjustable resistance means 26 as in the Fig. l embodiment.

In the operation of the system shown in Fig. 2 the rotation of the dial I! from the oif" position, as shown, to a selected temperature setting will serve to position the movable arm 80 relative to the adjustable resistance means 24 and set the the relay 50 which may be traced as follows: line wire 30, wire 32, movable arm ll, adjustable resistance means 26, wire 34, wire 38, relay coil '2,

. wire ll and wire 40 through snap-switch 62 to the other line wire 42. The thermal resistance means 24 is connected in parallel mesh circuit with the relay coil 52 by wire 48 from one end of resistance means 24 to wire 40 and by wire 44 from the opposite end of resistance 'means 24 to wire 14.

Hence, the thermal resistance means 24 which is located in the unheated oven and has a relatively high resistancewhen cold will permit sunlcient current flow through the coil 52 to cause movement of the armature 54 into attracted relation relative to the contact 56. A circuit for the heating element 48 is then established and may be traced as follows: line wire 30, wire 82, wire ll, heating element 48, wire 66, armature l4, contact 56, wire 88 and wire 40 through snap-switch 82 to the other line wire 42. It will be observed that the circuit for the heating element 48 and relay armature 54 is in parallel with the series circuit'of the adjustable resistance means 26 and the parallel mesh of thermal resistance means 24 and relay coil 52.

The energization of the heating means 4| now occurs and the temperature of the oven consequently increases. When the temperature setting of the handle I!) is reached the thermal resistance means 24 is sufiiciently heated and its resistance decreases causing an increase in the circuit current and voltage drop across the adjustable resisttromagnetic valve It. The safety control for the electromagnetic valve II comprises electrically operated control means 12 of electromagnetic form. The control means 12 comprises a horseshoe magnet I4 and armature ll loosely mounted on a stem 18. A contact bar is mounted on the stem and is biased to a released position ance means 26. The increase in voltage drop across the adjustable resistance means 26 is augmented by the positive temperature coefficient of this element. Consequently, the voltage drop across the coil 52 is decreased and the armature 54 is released to open the circuit of the heating means 40. Upon cooling of the oven, the described action is reversed and as the temperature 1 sensed by the thermal resistance means 24 is below the corresponding setting of the handle I, the relay coil 52 is sufficiently energized to attract the armature 54 to begin another heating cycle.

In the embodiment disclosed in Fig. 3 of the drawings, the temperature control system is shown applied to a space-heating arrangement of the general type disclosed in United States patent to Strobel No. 2,406,925 dated September 3, 1946. In the patented arrangement, a single electromagnetic valve is used for condition and safety control in conjunction with a gaseous fuel burner. The safety control is effected by placing the electromagnetic valve under control of a thermoelectric device responsive to a flame at the pilot burner. The condition control for the electromagnetic valve in the patented arrangement takes the form of a room thermostat operated through a bimetallic spiral of conventional type. The present invention is more particularly directed to an improved form of condition control for the disclosed system and, accordingly, only brief reference will be made to the parts fully described and shown in the aforementioned patent.

A main fuel burner i0 is supplied with gaseous fuel by the main fuel pipe l4 in which a main fuel shut-off cook 10 is provided. The flow of fuel in the pipe .I 4 is also under the control of the electromagnetic valve I! which is biased to closed together with the armature l. by the coil spring .2. In the released position of the contact bar ll fixed contact pairs 83, 44 associated with the bar I. are disengaged.

The electromagnetic device 12 is resettable by a second electrically operable means I. of electromagnetic form. The resetting means comprises a magnet plunger 88 for operating apivoted lever carrying a third pair of switch contacts 02 associated with the electromagnetic device 12. The arrangement is such that upon attraction of the magnet plunger 88 the lever 90 will be rotated on its pivot causing disengagement of the contact pair I2. resetting of the armature l0 and closing of '.the contact pairs l3, l4.

Thermoelectric means in the form of a thermocouple 4 is connected to a winding 96 for the horseshoe magnet 14.

a flame at the pilot burner 12 and will generatesumcient energy to retain the armature the resetting means 88. The operating coil 9! for the'inagnet plunger .8 takes the form of a primary coil of a transformer I". The secondary of transformer I" is connected to electric igniting means I32 for the pilot burner II. The flow of energy to the primary 98 is controlled by a thermal switch I04 having normally closed contacts "5, I" and a bimetallic operating arm Ill having a heater coil H0 associated therewith.

The foregoing description includes the essential parts of the patented arrangement which correspond to the parts disclosed herein. As previously indicated, the condition control for the system is different from that disclosed in the patent. The novel condition control device of this invention is located in a space in which the temperature is to be controlled, such as a room, and

embodies parts similar to those described in connection with the Figs. 1 and 2 embodiments of this invention.

The condition control may be mounted in a casing as indicated in broken lines in the drawing and which is mounted on the wall or other appropriate location in the space in which the temperature is to be controlled. The resistance means 24 and adjustable resistance means 26 are contained within the casing and the movable arm 2! is arranged for adjustment from the exterior thereof. The arrangement also includes the relay 50 having the coil 52 and armature 54 cooperable with the fixed contact 55. The relay 50 is adapted to control operation of the solenoid valve 18 and, thus, the flow of fuel to the main burner it. As in previous instances, the circuit connections will be described as the following description of the operation proceeds.

In the operation of the arrangement disclosed in Fig. 3, the main burner I0 is brought to the operating condition shown by closureof a double-pole main switch H2 when the movable arm 23 isset at a desired temperature setting on the indicia (not shown) and when the main fuel cock II is rotated to open position. Fuel is supplied to' the pilot burner l2 at this time but the The thermocouple 94 is iollows: line wire 80, one pole of main switch II2, wire II4, contacts I05, I06, wire II6, winding 88 and wire N8 through the other pole oi main switch II2 to line wire 42. The magnet plunger 88 is attracted and at the same time the igniter I02 is energized through the secondary of the transformer I00. Consequently, fuel flowing from the pilot burner I2 is ignited and the thermocouple 94 commences to become heated.

Owing to attr ction of the magnet plunger 88 the electroma netic device I2 is reset, the contact pair 82 is opened, and a circuit for the heater coil H8 is established as follows: line wire 80, one pole of main switch II2, wire II4, wire I20, winding IIO, wire I22, contact pair 83, wire I24 and wire H8 through the other pole of main switch II2 to line wire 42. After a predetermined period, the bimetal arm I08 warps to open the normally closed contact pair I05, I06. Thus, the winding 88 for the magnet plunger 88 is deenergized for the duration of the running period. As the lever 90 is then rotated counterclockwise by dropping of the magnet plunger 88, the third contact pair 92 is closed and a circuit for the solenoid valve I8 is established providing, however, that the relay 50 is operative to maintain the armature 54 in engagement with the fixed contact 56 thereof. This would be the case in the event that the room temperature is below the setting of the movable arm 28 and the resistance means 24 is consequently of normal high resistance. The circuit of the relay coil 52 has been established as soon as the main switch II2 was closed, as follows: line wire 30, one pole of main switch I I2, wire 32, arm 28, resistance means 26, wire 34, wire 36, coil 52, wire 38, and wire 40 through the other pole of main switch II2 to line wire 42. The thermal resistance means 24 is connected in parallel mesh circuit with the relay coil 52 by wire 46 from one end of resistance means 24 to wire 40 and by wire 44 from the opposite end of resistance means 24 to wire 34.

Under such conditions the circuit for the solenoid valve I8 may be traced as follows: line wire 42 to one pole of switch II2, wire 40, wire 68, contact 56, relay armature 54, wire I26, contact pair 84, wire I28, contact pair 92, wire I30, coil of solenoid valve I8 and wire I32 through the other pole of main switch II2 to line wire 38.

The fuel flowing tothe main burner I due to opening of the solenoid valve ignited by the flame at the pilot burner I2. The temperature in the space where the resistance means 24 is located will thus increase until the temperature corresponding to the setting of the adjustable resistance means 26 is reached. The resistivity of the resistance means 24 will then decrease sufliciently to cause an increase in the voltage dr, p across the adjustable resistance means 26 1" nd a consequent decrease in voltage drop acros the relay coil 52. Thus, at a definite temperature of the resistance means 24, the armature 54 is released to open the circuit of the solenoid valve I8 and shut-off the flow of fuel to the main burner I0.

The system will recycle when the relay coil 52 is again energized sufficiently to attract the armature 54 upon cooling of the space where the thermal resistance means 24 is located. The device thus operates as a room thermostat to control the main burner I0 as long asthe pilot burner I2 is producing a flame. As in the aforementioned patent, the extinguishment of the I8 will be' pilot burner flame will cause cooling of the thermocouple 94 and deenergization of the winding 86. The opening of the contact pairs 88, 84 by consequent release of the armature 18 serves to open the circuit of the solenoid valve I8 and the heater coil IIO. However, the contacts I05, I06 do not close until after a purge period, the length of which is determined by the time required b the bimetal I08 to cool.

It will be apparent that the relay 50 disclosed in the Fig. 3 embodiment could be dispensed with and the solenoid valve I8 controlled directly by the action of the thermal resistance means 24 as in the embodiment disclosed in Fig. 1. Thus, referring now more particularly to Fig. 4, a portion of the control syetem disclosed in Fig. 3 is illustrated in order to clarify the alternative arrangement. Thus, the alternative circuit for the solenoid valve I8 may be traced as follows: from the line wire 30, one pole of main switch I I2, wire 32, movable arm 28, adjustable resistance means 26, wire I34, contact pair 84, wire I28, contact pair 92, wire I30, coil of solenoid valve I8 and wire 48 through the other pole of main switch II2 to line wire 42.

The resistance means 24 is connected in parallel mesh circuit with the coil of the solenoid valve I8 by wire 44 connected at one end to wire 36 and at the opposite ends to one end of resistance means 24 and by wire 46 connected at one end to wire 40 and at the opposite end to the other end of the resistance means 24. The arrangement is thus somewhat similar to the Fig. 1 embodiment inasmuch as the adjustable resistance means 26 is connected in series circuit with the parallel mesh of resistance means 28 and the coil of the solenoid valve I8. As the operation of the embodiment disclosed in Fig. 4 will be clear from the description of the Fig. 3 and other embodiments, it will not be repeated.

It will be understood that many changes may be made in the details of construction and arrangement of parts without departing from the present invention so that this description and accompanying drawings are intended by way of illustration only and are not to be construed in a limiting sense.

I claim:

1. A control system for fuel burners having main and pilot burners, comprising in combina-- tion, electrically operable means movable between operative positions for controlling the supply of fuel to the main burner, thermomagnetic switch means responsive to a flame at the pilot burner and operable to a first position for causing operation of said electrically operable means to one of said positions, and to a second position for causing operation thereof to another said position, means for resetting said thermomagnetic switch means in said first position including second electrically operable means, means responsive to movement of said thermomagnetic switch means to said first position for causing deenergization of said second electrically operable means, thermal resistance means located to respond to a temperature condition to be controlled by varying in resistivity in accordance with variations in said condition and being electrically connected to said first electrically operable means, and resistance means velectrically connected to said thermal resistance means and being adjustable over a range of resistivity settings corresponding to selected values of said condition, the arrangement being such that at a predetermined resistivity of said thermal resistance means the energy value supplied to said 9 electrically operable means is varied suiilciently for operation thereof between said positions.

2. A control system for fuel burners having main and pilot burners, comprising in combination, electromagnetic valve means movable between open and closed positions for controlling the supply of fuel to the main burner, thermomagnetic switch means in series circuit with said electromagnetic valve means and responsive to a flame at the pilot burner for opening said circuit in the absence of said flame, means for resetting said thermomagnetic switch means for closing said circuit including a relay having a coil and an armature mechanically connected to said thermomagnetic switch means for operating the same, means for opening the circuit of said relay coil upon said circuit closing operation of said thermomagnetic switch means, a second relay having a coil and an armature connected for opening and closing said circuit independently of said thermomagnetic switch means, first resistance means located to be heated in a space in which the temperature is to be controlled and by varying in resistivity in accordance with variations in said temperature and being connected in parallel mesh circuit with said second relay coil, and second resistance means connected in series circuit with said mesh and being adjustable over a range of resistivity settings corresponding to selected values of said temperature, said first and second resistance means being cooperable for causing energy of sumcient value to be supplied to said second relay coil when said first resistance means is unheated for operation of said second relay armature for closing said circuit and causing operation of said valve means to open position, and for reducing said energy below said suflicient value for operation of said second relay armature for opening said circuit and causing operation of said valve means to closed position when said first resistance means is heated at substantially the selected temperature of said space.

3. A control system for fuel burners having main and pilot burners, comprising in combination, electromagnetic valve means movable between open and closed positions for controlling 10 the fuel supply to the main-burner, thermomagnetic switch means in series circuit with said electromagnetic valve means and responsive to a flame at the pilot burner for opening said circuit in the absence of said flame, means for resetting said thermomagnetic switch means for closing said circuit including a relay having a coil and an armature mechanically connected to said thermomagnetic switch means for operating the same, means for opening the circuit of said relay coil upon said circuit closing operation of said thermomagnetic switch means, first resistance means located to be heated in a space in which the temperature is to be controlled and by varying in resistivity in accordance with variations in said temperature and being connected in parallel mesh circuit with said electromagnetic valve means, and second resistance means connected in series circuit with said mesh and being adjustable over a range of resistivity settings corresponding to selected values of said temperature, said first and second resistance means being cooperable for causing energy of sufllcient value to be supplied to said electromagnetic valve means when said first resistance means is unheated for causing operation to open position, and for reducing said energy below said suflicient value for operation of said electromagnetic valve means to closed position when said first resistance means is heated at substantially the selected temperature of said space.

CHARLES K. STROBEL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,528,053 Hands Mar. 3, 1925 2,406,925 Strobe] Sept. 3, 1946 2,487,556 Jenkins Nov. 8, 1949 2,545,353 Gund Mar. 13, 1951 FOREIGN PATENTS Number Country Date 557,707 Great Britain Dec. 1, 1943 

